In Vivo Characterization of Platinum(II)-Based Linker Technology for the Development of Antibody-Drug Conjugates: Taking Advantage of Dual Labeling with ^195mPt and ⁸⁹Zr.

Abstract

Linker instability and impaired tumor targeting can affect the tolerability and efficacy of antibody-drug conjugates (ADCs). To improve these ADC characteristics, we recently described the use of a metal-organic linker, [ethylenediamineplatinum(II)]²⁺, herein called <i>Lx</i> Initial therapy studies in xenograft-bearing mice revealed that trastuzumab-<i>Lx</i>-auristatin F (AF) outperformed its maleimide benchmark trastuzumab-mal-AF and the Food and Drug Administration-approved ado-trastuzumab emtansine, both containing conventional linkers. In this study, we aimed to characterize <i>Lx</i>-based ADCs for in vivo stability and tumor targeting using ^195mPt and ⁸⁹Zr. <b>Methods:</b> The γ-emitter ^195mPt was used to produce the radiolabeled <i>Lx</i> [^195mPt]<i>Lx</i>⁸⁹Zr-Desferrioxamine (⁸⁹Zr-DFO) was conjugated to trastuzumab either via [^195mPt]<i>Lx</i> (to histidine residues) or conventionally (to lysine residues) in order to monitor the biodistribution of antibody, payload, and linker separately. Linker stability was determined by evaluating the following ADCs for biodistribution in NCI-N87 xenograft-bearing nude mice 72 h after injection: trastuzumab-[^195mPt]<i>Lx</i>-DFO-⁸⁹Zr, trastuzumab-[^195mPt]<i>Lx</i>-AF, and ⁸⁹Zr-DFO-(Lys)trastuzumab (control), all having drug-to-antibody ratios (DARs) of 2.2-2.5. To assess the influence of DAR on biodistribution, ⁸⁹Zr-DFO-(Lys)trastuzumab-<i>Lx</i>-AF with an AF-to-antibody ratio of 0, 2.6, or 5.2 was evaluated 96 h after injection. <b>Results:</b> Similar biodistributions were observed for trastuzumab-[^195mPt]<i>Lx</i>-DFO-⁸⁹Zr, trastuzumab-[^195mPt]<i>Lx</i>-AF, and ⁸⁹Zr-DFO-(Lys)trastuzumab irrespective of the isotope used for biodistribution assessment. The fact that <i>Lx</i> follows the antibody biodistribution indicates that the payload-<i>Lx</i> bond is stable in vivo. Uptake of the 3 conjugates, as percentage injected dose (%ID) per gram of tissue, was about 30 %ID/g in tumor tissue but less than 10 %ID/g in most healthy tissues. Trastuzumab-[^195mPt]<i>Lx</i>-AF (DAR 2.2) showed a tendency toward faster blood clearance and an elevated liver uptake, which increased significantly to 28.1 ± 4.2 %ID/g at a higher DAR of 5.2, as revealed from the biodistribution and PET imaging studies. <b>Conclusion:</b> As shown by ^195mPt/⁸⁹Zr labeling, ADCs containing the <i>Lx</i> linker are stable in vivo. In the case of trastuzumab-<i>Lx</i>-AF (DARs 2.2 and 2.6), an unimpaired biodistribution was demonstrated.